1. Field of the Invention
Embodiments herein present a structure, method, etc. for making high density MOSFET circuits with different height contact lines.
2. Description of the Related Art
Metal-oxide semiconductor field-effect transistors (MOSFETs) are a special type of field-effect transistor (FET) that work by electrically varying the voltage in a channel to control charge carriers (electrons or holes) flow along the channel. The charge carriers enter the channel at the source, and exit via the drain. The charge carriers flow in the channel is controlled by the voltage on an electrode, or the gate, which is located physically between the source and the drain and is insulated from the channel by an extremely thin layer of metal oxide.
There are two ways in which a MOSFET can function. The first is known as depletion mode. When there is no voltage on the gate, the channel exhibits its maximum conductance. As the voltage on the gate increases (either positively or negatively, depending on whether the channel is made of P-type or N-type semiconductor material), the channel conductivity decreases. The second way in which a MOSFET can operate is called enhancement mode. When there is no voltage on the gate, there is in effect no conductive channel, and the device does not conduct. A conductive channel is produced by the application of a voltage to the gate. The greater the gate voltage, the better the device conducts.
The MOSFET has certain advantages over the conventional junction FET, or JFET. Because the gate is insulated electrically from the channel, no/small current flows between the gate and the channel, no matter what the gate voltage (as long as it does not become so great that it causes physical breakdown of the metallic oxide layer). Thus, the MOSFET has practically infinite impedance. This makes MOSFET circuits useful for power amplifiers. The devices are also well suited to high-speed switching applications. Some integrated circuits (ICs) contain tiny MOSFET circuits and are used in high performance ICs such as computers or servers.
Despite its advantages, MOSFET circuits are susceptible to source and drain blockage due to aggressive scaling of semiconductor devices. Specifically, the source and drain areas of MOSFET circuits are becoming increasingly smaller, which can lead to blockage of the source and drain areas by spacers. Spacers are necessary components of MOSFET circuits, which function to isolate gate contact and source and drain contacts. In addition, spacers also passivate sidewalls of gate stacks. However, spacers are not always useful and in some cases have disadvantages. For example, the gate stacks are often used as contact lines to connect different devices. Spacers on the contact lines can increase the difficulty forming contact between the contact lines and the devices needed to contact. Moreover, these undesirable spacers occupy space and then reduce density of ICs. This increases the costs of manufacturing of ICs. Therefore, it is readily apparent that there is a need for a structure, method, etc. for making high density MOSFET circuits with different height contact lines, wherein spacer blockage of contact areas is avoided. In this case, undesirable spacers can be avoided to save space or reduce costs for IC design.